In many processes, for example, in the area of process engineering, chemistry, or mechanical engineering, a well-defined gas mass, in particular an air mass, must be supplied. This includes in particular combustion processes, which take place under controlled conditions. One important example is the combustion of fuel in internal combustion engines of motor vehicles, in particular having downstream catalytic emission control. Different types of sensors are used for measuring the air-mass flow.
One conventional sensor type is the so-called hot-film air-mass meter (HFM), a specific embodiment of which is described, for example, in German Patent Application No. DE 196 01 791 A1. In such hot-film air-mass meters, a thin sensor diaphragm is usually applied to a sensor chip, for example, a silicon sensor chip. At least one heating resistor, surrounded by two or more temperature measuring shunts, is usually situated on the sensor diaphragm. In an air flow guided over the diaphragm, the temperature distribution changes, which in turn may be detected by the temperature measuring shunts. An air-mass flow may thus be determined, for example, from the difference between the resistances of the temperature measuring shunts. There are different variants of this conventional sensor type. Such sensors are used, for example, directly in the intake tract of an internal combustion engine or in a bypass channel. One exemplary embodiment in which a sensor chip is used in a bypass channel is described, for example, in German Patent Application No. DE 103 48 400 A1.
A problem described, for example, in German Patent Application No. DE 101 11 840 C2, with this type of sensor is, however, that contamination of the sensor chip, by oil for example, often occurs. The sensor chip is normally used directly in the intake tract of the internal combustion engine or in a bypass channel to the intake tract of the internal combustion engine. Oil may deposit on the sensor chip and, in particular, on the sensor diaphragm during operation or shortly after the internal combustion engine has been shut off. This oil deposit may result in undesirable effects on the measuring signal of the sensor chip, in particular because an oil film affects the thermal conductivity of the sensor chip surface, which results in corruption of the measuring signals or a signal drift.
Also, liquids are subject to a force in the direction of the colder regions in the presence of a temperature gradient (see, for example, V. G. Levich, “Physicochemical Hydrodynamics,” Prentice-Hall, N.J., 1962, p. 384 seq.) This is one of the reasons why, when operating a thermal air-mass flow meter at the border region of the heated measuring areas, liquids such as oil accumulate and over time result in a drift of the measuring signal of the hot-film air-mass meter. The air flow drives the liquid droplets and other contaminants on the surface up to the boundary of the heated measuring area, at which a stronger temperature gradient appears. The strong temperature gradient exerts a force opposite to the force exerted by the air flow. Liquid droplets thus accumulate on the boundary line, which, when they reach a certain size, may be entrained again by the air flow to then contaminate the surface of the measuring area. In addition to the oil droplets, other contaminants (e.g., dust) also reach the surface of the measuring area due to this effect.
This effect, whereby oil and other contaminants are driven onto the surface of the measuring area in irregular intervals, causes short-term and unpredictable signal instabilities of the hot-film air-mass meter in particular. These are caused, in particular, by the sporadically occurring contamination modifying the thermal conductivity of the surface of the measuring area, whereby a previously performed calibration of the hot-film air-mass meter becomes invalid. In addition to short-term changes, longer-lasting changes of the signal characteristic of the hot-film air-mass meter may also occur in particular if the contamination driven onto the measuring surface adheres there for a longer time.
Another problem arising from contamination accumulating in particular on the boundary of the measuring area is the effect on the flow dynamics of the hot-film air-mass meter. The hot-film air-mass meter is calibrated before being put in service, the calibration being based on a certain flow characteristic of the air-mass flow over the surface of the hot-film air-mass meter. If, however, contamination, in particular a liquid wall, accumulates on the boundary surface of the measuring area during operation, it also affects the velocity profile of the air-mass flow over the measuring surface and thus the temperature profile. Since, however, the heat transport on the measuring surface is a function of the shape of the velocity profile and temperature profile, this results in a signal drift of the hot-film air-mass meter.